Low discharge rate alkaline battery

ABSTRACT

A battery is provided comprising an aluminum anode, an alkali aqueous  solon of controlled concentration and temperature and a second electrode operates with high anode efficiency.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to an aluminum based battery and moreparticularly to an aluminum based battery having a controlledconcentration of alkaline electrolyte wherein discharge rate is low andthe anode efficiency is high. Presently, a high power density primarybattery based on aluminum and silver oxide alkaline half cells providesufficient energy for vehicle propulsion. The major advantage of thiselectrochemical system is the extraordinary current densities, on theorder of 1600 mA/cm, which are readily achieved. These high currentdensities are indicative of facile electron transfer in both the anodicand the cathodic redox couples. The high densities in the alkalinealuminum/silver oxide cathodic couple may be attributed to the anomaloussolid phase mobility of Ag⁺. Unlike other cations, the silver cationtravels rapidly not only through the liquid phase but also through thesolid phase of its salts. Therefore, as AgO is reduced, Ag⁺ cancontinually travel to the electrode interface, preventing surfacepassivation and permitting continuous facile electron transfer.

(2) Description of the Prior Art

Prior to the present invention, it had been demonstrated that thealuminum-silver oxide battery was capable of operating with aluminumefficiencies above 90 percent during discharge at current densities inthe range of 500 to above 1500 milliamps per square centimeter usingsodium or potassium hydroxide electrolytes at above 70° C. and atconcentrations above 4 molar. However, under those same operatingconditions, aluminum efficiencies drop to less than 50 percent when thecurrent density was lowered into the range of 30 milliamps per squarecentimeter. Certain underwater vehicles require high energy densitybatteries capable of operating at very low to very high drain rates (30to 1500 milliamps per square centimeter). The aluminum and silver oxidebattery system would be a useful energy source for such vehicles only ifits energy density at low rates could be improved. The corollaryrequirement was that it must operate with high aluminum efficiencies(low corrosion rates) at low current densities, otherwise corrosion ofthe aluminum produces excessive gas and reduces the energy density byusing excessive electrolyte and aluminum.

Examples of aluminum based alkaline batteries are disclosed in U.S. Pat.Nos. 3,953,239; 4,107,406 and 4,150,204.

It would be desirable to provide an aluminum based battery capable ofproducing high voltages and operating at high efficiencies whenfunctioning at low discharge rates.

SUMMARY OF THE INVENTION

This invention provides a battery which comprises an aluminum anode, analkali electrolyte of controlled concentration and containing sodiumstannate (e.g., 10-20 grams per liter) and a silver oxide cathode. Thealkali electrolyte is dissolved in seawater, distilled water or tapwater wherein the alkali concentration is between about 1 and 3 molar.The performance of the battery is enhanced by high temperatures, fromabout 45° to 55° C. and modest electrolyte flow velocities of about 200to 800 centimeters per minute. When operated at low discharge rates ofbetween about 30 and 100 milliamps per centimeter square (mA/cm²), highcell voltages of between 1.85 and 2.074 volts per cell and highoperating efficiencies of 85% or higher are obtained. It has beendiscovered that these high efficiencies are not obtained at the lowerrates (less than about 50 mA) when the alkali concentration in theelectrolyte exceeds about 3 molar or when temperatures exceed about 55°C.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention and of the attendantadvantages thereto will be readily appreciated as the same becomesbetter understood by references to the following detailed descriptionwhen considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic diagram of a system utilizing the batterystructure of this invention.

FIG. 2 is a graph of anode operating efficiencies and cell voltagesobtained and projected as a function of current density and alkaliconcentration at 50° C. as described in Example 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The anode of the battery of this invention can be pure

aluminum which is readily available at a purity of at least about 99.99%or an aluminum alloy depending on the electrolyte composition. Thebattery of this invention is based upon the following electrochemicalreactions:

    ANODE: 2 Al+8 OH.sup.- →2 AlO.sub.2.sup.- +4 H.sub.2 O+6e.sup.-(1)

    CATHODE: 3 AgO+3H.sub.2 O+6e.sup.- →3Ag+6OH.sup.-   (2)

    OVERALL BATTERY: 2 Al+3AgO+2OH.sup.-→ 2AlO.sub.2.sup.- +3Ag+H.sub.2 O(3)

The aluminum alloy has composition which permits the electrochemicalreaction at the anode to proceed as set forth above by Equation 1.Representative suitable alloys include the aluminum plus magnesium,(0-0.8%), tin (0-0.15%), and gallium (0-0.05%) or the like.

As set forth above, the battery of this invention is operated at anefficiency greater than about 85%. "Efficiency" as used herein isdefined as 100 times the amount of aluminum consumed electrochemicallydivided by the total amount of aluminum consumed electrochemically andby corrosion.

The electrolyte utilized in the battery of this invention comprises analkali aqueous solution wherein the concentration of alkali is betweenabout 1 and 3 molar preferably between about 1.5 and 2.5 molar. Suitablealkaline compositions which can be utilized include potassium hydroxideor sodium hydroxide. The alkali solution can be formed from distilledwater, tap water or seawater.

A second electrode is required to complete the battery circuit and isformed of silver peroxide capable of causing the oxidation of water.Representative suitable materials for forming the second electrode alsoinclude silver oxide, hydrogen peroxide, oxygen, nickel oxyhydroxide orthe like. Silver peroxide comprises the preferred second electrode.

In use, it is preferred to operate the battery of this invention atelevated temperatures on the order of 50° C. The temperature isconveniently controlled by flowing the electrolyte between the batteryhousing and a heat exchanger by any conventional means. An effectivemeans for operating the battery of this invention is shown in FIG. 1.The battery 10 includes an aluminum anode, the second electrode and analkali electrolyte having an alkali concentration preferably betweenabout 1.5 and 2.5 molar. The electrolyte is removed from the housing 10through conduit 12 by means of pump 14 and is cycled to heat exchanger16 through gas-liquid separator 18 and valve 20. The electrolyte fromheat exchanger 16 is returned to battery 10 through electrolytereservoir 22 and conduit 24. Make-up electrolyte is supplied from tank26. Hydrogen gas is removed from separator 18 through conduit 30.Make-up seawater is added to reservoir 22 through conduit 28. Wasteelectrolyte is removed from reservoir 22 through conduit 31.

The following example illustrates the present invention and is notintended to limit the same.

EXAMPLE I

This example illustrates the utility of the aluminum based alkalibattery of this invention. The electrolyte utilized in the batterycomprised a NaOH aqueous solution containing 20 grams of sodium stannateper liter, a silver peroxide second electrode and a 99.99% aluminumanode alloyed with magnesium, tin, and gallium. Measurements ofefficiency and cell voltage were made at varying current densities.

A graph of measured efficiency and discharge voltage as a function ofcurrent density and alkali concentration at 50° C. is shown in FIG. 2.As shown in FIG. 2, higher anode efficiencies are obtained at loweralkali concentrations, especially at current densities below 50 mA/cm²while obtaining high cell voltages.

A similar cell with all parameters the same except operating with a moreconcentrated alkali electrolyte, is capable of producing the same highvoltage but with anode efficiencies only in the range of 50 to 70%. Atcurrent densities above 50 mA/cm² a wider latitude in the range oftemperatures and alkali concentrations provide acceptable efficiencies.

In light of the above, it is therefore understood that within the scopeof the appended claims, the invention may be practiced otherwise than asspecifically described.

What is claimed is:
 1. A battery comprising:a housing; an aluminum anodewithin said housing; an electrode capable of oxidizing water within saidhousing; a 1 to 3 molar alkali electrolyte solution; and a heatexchanger means for controlling the temperature of said electrolytesolution.
 2. The battery of claim 1 wherein said electrolyte is sodiumhydroxide.
 3. The battery of claim 1 wherein said electrode is silverperoxide.
 4. The battery of claim 2 wherein said electrode is silverperoxide.
 5. The process for producing an electrical current whichcomprises providing a circuit between the cathode and the anode of thebattery of claim 1 and wherein said electrolyte solution is maintainedat a temperature between about 45° C. and 55° C.
 6. The process of claim5 wherein said electrode is silver peroxide.
 7. The process of claim 5wherein said electrolyte is sodium hydroxide and said electrode issilver peroxide.
 8. The process of claim 5 wherein said alakalielectrolyte solution is 1.5 to 2.5 molar.
 9. The process of claim 6wherein said alkali electrolyte solution is 1.5 to 2.5 molar.
 10. Theprocess of claim 7 wherein said alkali electrolyte solution is 1.5 to2.5 molar.